Cystic fibrosis (CF) is a life-limiting condition that begins in early childhood, affecting 30,000 people in the United States. I am an Assistant Professor of Pediatrics at the University of Colorado Denver (UCD), committed to a career as an independent translational investigator in order to improve the lives of children with CF. My research focuses on the role of lung infections and host-response during pulmonary exacerbations. This application for a K23 Mentored-Career Development Award addresses three critical areas that will allow me to develop into an independent investigator: (1) approaches to microbiome analyses, particularly related to analysis of high-dimensional datasets, (2) the study of host-response mechanisms in CF, and (3) training in clinical investigation. My mentoring team is ideally suited to address these areas. Dr. Frank Accurso, a CF researcher for over 20 years, is in an expert in host-response in CF. Dr. Stanley Szefler has extensive experience in airway inflammatory biomarkers and clinical investigation in pediatric asthma, and Dr. Norman Pace is an expert in microbiome analyses. UCD offers an outstanding training environment. My career development includes course work through UCD, research training through the NIH supported Colorado Clinical and Translational Sciences Institute, and an externship with the CF Foundation Therapeutics Development Network Coordinating Center in Seattle, Washington. Exceptional internal and external review committees will evaluate my progress. My research proposal focuses on pulmonary exacerbations (PEx), known to contribute greatly to progressive lung function decline and decreased quality of life. The etiology of PEx is incompletely understood, particularly in the pediatric age range; however, bacterial infection and host-response contribute. My long term goal is to improve treatment of PEx by targeting newly identified bacterial pathogens or through modulation of the microbiome. Our specific aims are to (1) determine the relationship between the microbiome, host-response and lung function at the onset of PEx, (2) determine the changes that occur in the microbiome, host-response and lung function with treatment and recovery from PEx, and (3) examine the relationship between Prevotella, a bacterial genera with pathogenic potential, and response to treatment of PEx. We propose to prospectively collect blood and sputum samples from 50 well-characterized CF subjects serially during a PEx and at a clinically stable follow-up visit. Analyses of sputum samples will include standard culture and molecular microbiology approaches (sequencing and quantitative PCR). Inflammatory cytokines, proteases, and markers of lung tissue injury will be measured from blood and sputum. Circulating protein biomarkers will also be measured using a novel aptamer-related approach; pathway analysis of these proteins will be used to identify underlying mechanisms of host-response. Taken together, these studies will provide considerable new knowledge concerning PEx in CF and allow me to apply for R01 funding in the third year of my award.